Optical beam steerers are known in the prior art. A liquid crystal-based optical phased array (LC-OPA) may be the current the state-of-the-art in optical beam steering devices. See P. F. McManamon et. al., Proceedings of IEEE, Vol. 40, No. 2 (1996) p. 268. It consists of a liquid crystal (LC) cell with one-dimensional patterned transparent conductor strips in which each strip defines an element of the linear array. For beam steering in the two azimuthal and elevational directions, two such LC cells are arranged in orthogonal orientations.
LC-OPA is a fairly mature technology with very low power consumption due to the capacitive nature of the liquid crystal. However, the disadvantage of the LC-OPA is its slow steering speed (10's of ms range) which is due to the slow response time of the LC-based phase shifting elements. Another disadvantage of liquid crystals is their limited temperature operation range. At low temperatures (<0° C.) the LC response time significantly degrades due to its increased viscosity, while at higher temperatures (>50° C.) it becomes isotropic and hence loses functionality. Consequently, for practical purposes, the operating temperature of the LC-OPA should be externally controlled, which further adds to its complexity of having two separate LC cells assembled in tandem for 2-D beam steering.
The other problem with LC-OPA is the presence of grating lobes in the steered beam, which not only severely reduces the optical efficiency of the phased array but also requires complex signal detection circuitry. In order to eliminate grating lobes in phased arrays, the spacing between array elements must be less than the wavelength of the steered optical beam. In LC-OPAs, the array elements (strip width) are in the range of 5 to 10 μm, and hence larger than the wavelength of the optical beam in the visible and near-IR region. Reducing the strip width below 5 μm results in a significant field fringing effect, and hence decreased electro-optic efficiency, since the thickness of the LC cell is about 4 μm.
Another prior art beam steering approach is based on the use of integrated AlGaAs waveguide arrays on a GaAs substrate in which each array element is a tunable phase shifter. See F. Vasey, et. al., Applied Optics, Vol. 32 (1993) p. 3220. The phase tuning is achieved via the linear electro-optic effect in the material by forming a heterojunction barrier with a low resistivity transparent conductor (indium-tin-oxide) cladding layer.
The main problem with integrated AlGaAs waveguide array approach is the rather limited phase delay achievable with these tunable waveguides due to a weak electro-optic effect. Waveguide lengths of more than 3 mm are required in order to obtain a 2π phase delay at 850 nm. Also, similar to the LC-OPA described above, this phased array will also have grating lobes, since the minimum width of the waveguides (about 2.5 μm) is a factor of two to three larger than the operating wavelength.
In contrast, a phased array beam steerer based on the disclosed tunable metamaterial disclosed hereon does not suffer from grating lobes since the spacing of the waveguide array can be as small as the unit cell dimension, which is about ¼ to ⅓ of the wavelength. Furthermore, since tunable phase shifts of 0-2π can be achieved between adjacent array elements with array spacing of less than half the wavelength, the steering angle of this novel beam steerer can approach 0-90° in both azimuthal and elevational directions, which is significantly larger than those achievable with the LC-OPA and the AlGaAs waveguide devices mentioned above.